The content and distribution of water in wood affect its performance. So far, most technologies to measure water content/distribution can only be applied to small wood samples. A method to measure the water content inside large timber is urgently needed, given that information on water content/distribution inside the harvested timber is often required before the timber can be processed to products. In this work, a mini cylindrical NMR sensor (i.e., 28 mm in diameter and 60 mm in length) for measuring water content inside large timber was designed. The sensitive volume is a coaxial ring just outside of the sensor. For measurement, the sensor needs to be placed inside a hole drilled on the timber, which has a similar diameter with the sensor. By moving the sensor around the wall of the hole, the NMR parameters of the timber could be measured. Preliminary experimental results showed that the sensor can detect signals from both free water and the bound water in large timber, and such signals can be used to calculate relative water contents.